The present invention relates to a process for bringing out a high contrast in the new wood part near the surface relative to the old wood part in the grain pattern of a piece of wood which has little contrast in the original state, preferably an almost uniformly light piece of wood, as previously known, for example, by the so-called flaming of wood in wood technology.
The conventional processes of wood flaming take place, however, in two stages. The wood is first charred near the surface, and the carbonized layer is then being brushed off again. In the course of brushing the softer new wood part is removed to a greater extent and becomes light in appearance again, while the harder old wood part is removed to a lesser extent and consequently stands out not only in relief, but also contrasting highly in color relative to the new wood part. This type of wood treatment is suitable for a rustic appearance of wood items, where very strong color contrasts and also a very strong surface shaping of the grain pattern are important. In the case of items where a smooth surface is desired this browning technique is not possible.
It is also conceivable for the flaming to be carried out with a relatively low heat intensity, so that browning is limited to the soft new wood parts, while the harder old wood parts still remain pale. The disadvantage of this process is, however, that the browning pattern becomes very uneven on account of an uneven convection which is associated with the flame formation and which, in some circumstances, can be increased even further by a veneer sheet treated in this way becoming wavy.
Contact browning processes for bringing out a highly contrasting wood grain in naturally pale wood items are also known. Hot-stamping is, however, generally used in these cases. A new, stereotyped grain pattern is hot-stamped in this way into a uniformly light wood with little surface structure, in the course of which not only a surface structuring but a differing heat supply and also browning, to a greater or lesser extent, are achieved through a selected unevenness in the temperature distribution and/or the surface pressing during the stamping operation. The disadvantage here is that only a stereotyped grain pattern corresponding to the stamping die can be achieved, but further that the natural grain of the wood itself cannot be brought out by the process.
Another known contact browning process operates with heated rollers between which the veneer sheet is passed. In the course of this passage, the softer new wood parts are browned to a greater extent than the harder old wood parts. The disadvantage of this process is that the grain pattern emerges locally in varying different degrees, which may be attributed not only to locally different thicknesses of the veneer sheet, but also to local differences in moisture within the veneer sheet. In addition, original slight stress marks of the wood which initially remained invisible become clear to the eye in the browned grain pattern. At best, very uniformly thick veneer sheets which have a very finely worked surface can be treated in this way. Despite everything, only a very low color contrast is achievable.
An object of the present invention is, therefore, to develop a basic process such that, despite any waviness of the veneer sheet, fluctuations in thickness, moisture fluctuations or slight chatter marks, a uniform and intensive color contrast of the natural grain, together with the smooth final surface of the piece of wood, can still be achieved in a single operation.
This object has been achieved according to the present invention by making the heat application contact-free through infrared radiation in a stagnant ambient atmosphere. Due to the contact-free infrared irradiation, uniform grain patterns can be achieved even if there are fluctuations in thickness. A turbulence or convection pattern disturbing the uniformity of the browned grain pattern is not produced by the heat radiation. On account of the careful but still intensive heat application, a high intense color contrast can also be achieved.